1. Field of the Invention
The present invention relates to a sheet feeder for an image forming apparatus and, more particularly, to a sheet feeder provided with a mechanism for transporting sheets successively at short intervals to an image transfer unit.
2. Description of the Prior Art
In feeding a sheet by a sheet feeder to the photosensitive member of an xerographic image forming apparatus, the sheet is held at a position immediately before the photosensitive member by a registration means to feed the sheet in phase with an image formed on the photosensitive member.
The Sheet feeder shown in FIG. 9 is disclosed in Japanese Patent Laid-open (Kokai) No. Sho 60-77051 (first reference). In this sheet feeder, a feed roller 2 is activated at time t.sub.1 to feed out a sheet 6 from a sheet feed tray 5. The feed roller 2 is disengaged from a driving mechanism at time t.sub.2, and then the sheet 6 is transported by transport rollers 8. Upon the detection of the passage of the leading edge of the sheet 6 by a sheet detector 12 disposed near a registration rollers 10 at time t.sub.3, the time required for the leading edge of the sheet 6 to arrive at the sheet detector 12 after the actuation of the feed roller 2 i.e. T.sub.d =t.sub.3 -t.sub.1, is calculated. A registration rollers 10 is activated at time t.sub.4 to deliver the sheet 6 to a photosensitive drum 14. A time difference .DELTA.T=T-T.sub.d, where T is a standard time in which the leading edge of the sheet 6 should arrive at the registration rollers 10, is calculated and the time t.sub.1 the feed roller into action is changed to t.sub.1 +.DELTA.T so that the next sheet 6 arrives at the registration roller 10 at correct time with respect to the angle of the photosensitive drum 14.
In feeding a sheet by a sheet feeder shown in FIG. 10, disclosed in Japanese Patent Laid-open (Kokai) No. Sho 61-51428(1986) (second reference), a feed roller 2 and a sheet separating roller 4-1 are activated in response to a sheet feed signal. Sheet detectors 12, 20 and 22 detect the leading edge of a sheet 6. The feed roller 2 is stopped upon the detection of the leading edge of the sheet 6 by the sheet detector 22. A transport speed V.sub.1 at which the sheet 6 has been transported between the sheet detectors 20 and 22 is calculated. The time when transportation of the sheet 6 is to be restarted is determined on the basis of the transport speed V.sub.1 and a transport speed V.sub.2 at which the preceding sheet 6 was transported between the sheet detectors 22 and 12 by a transport rollers 8; that is, the time when transportation of the sheet is to be restarted is adjusted on the assumption that this sheet 6 will be transported at the transport speed V.sub.1 as far as the transport rollers 8, and at the transport speed V.sub.2 from the transport rollers 8 to registration rollers 10 so that the sheet 6 arrives at the registration rollers 10 at the predetermined time. An estimated value for the transport speed V.sub.1 is used for calculating the time when transportation of the first sheet 6 is to be restarted. Thus, variations in the transport speed due to irregularity in the position of the leading edges of sheets stacked at a sheet stacking position is corrected to reduce gaps between the successive sheets required for compensating the variation of transport speed.
Japanese Patent Laid-open (Kokai) No. Sho 62-136442 (third reference) discloses the multistage sheet feeder shown in FIG. 11.
Referring to FIG. 11, a feed roller 2 and a sheet separating roller 4-1 are activated in response to a sheet feed command to feed out a sheet 6 from a sheet feed tray 5. Next, the sheet 6 is transported to transport rollers 26 of a copying machine 30 by transport rollers 8 capable of rotating at three different transport speeds. A sheet detector 24 disposed near the sheet inlet of the copying machine 30 detects the passage of the leading edge and the trailing edge of the sheet 6. If the sheet feed command requires feeding a plurality of sheets, the gap between the trailing edge of the preceding sheet and the leading edge of the succeeding sheet is calculated on the basis of detection signals provided by the sheet detector 24. If the calculated gap is different from a reference gap, a time gap for which the transport speed is to be changed is determined by calculation on the basis of the difference between the calculated gap and the reference gap, and then the transport speed is changed according to the result of calculation so that the sheets are transported at fixed gaps. The surface speed of the transport rollers is increased for the calculated time gap if the gap between the successive sheets is greater than the reference gap or the surface speed of the transport rollers is decreased for the calculated time gap if the gap between the successive sheets is shorter than the reference gap to feed the sheets at the fixed gaps to the copying machine.
A typical digital image forming apparatus, such as a digital copying machine, converts the image of an original document into digital image signals by a CCD line sensor or the like and stores the digital image signal as image data in a storage device. The image data is converted into light signals to form a latent image corresponding to the image of the original document on a photosensitive drum. Since the same image data can be used repeatedly to produce a plurality of copies of an original document, only a single cycle of the reading operation for reading the image of the original document is necessary to produce a plurality of copies of an original document. Since the digital image forming apparatus does not need any time for returning the image scanning unit to its original position, which is essential to a conventional copying machine with no image storage facility, the image can be written continuously on the photosensitive drum. Accordingly, if the image is written on the photosensitive drum at reduced gaps and the sheets are fed in synchronism with the image writing operation, the throughput of the copying machine, namely, the number of copies produced in a unit time, can be increased.
In some cases, a sheet is fed out from the sheet feed tray with a delay due to slipping of the feed roller relative to the sheet or, if two or more sheets are fed out by the feed roller from the sheet feed tray and one of the sheets is separated and fed by the sheet separating roller, the following sheet is fed out earlier because it is held dislocated from the sheet stacking position in the sheet transporting direction. Since the sheet feed timing thus varies, the prior art has problems that the time when the sheet arrives at the registration means varies and that collisions between successive sheets occur if the sheets are fed at short intervals.
The sheet feeder disclosed in the first reference corrects the timing of feeding out the succeeding sheet on the basis of the transport time required for transporting the preceding sheet. Accordingly, if the preceding sheet and the succeeding sheet are fed out under different feed-out conditions, for example, if the preceding sheet and the succeeding sheet are fed out together and the succeeding sheet is held by the sheet separating roller at a position dislocated from the sheet stacking position, the correction is ineffective and, consequently, the time when the sheet arrives at the registration rollers varies.
Although the sheet feeder disclosed in the third reference is capable of transporting the successive sheets at fixed gaps and of adjusting the gap between successive sheets to the reference gap on the basis of a measured gap, the sheet feeder is unable to control the transportation of the sheet so that the sheet arrives at a predetermined position at a predetermined time.
Accordingly, in feeding sheets successively by these prior art sheet feeders, each sheet must be delayed by the registration means disposed immediately before the photosensitive drum to absorb variations in the time when the sheet arrives at the registration means. Therefore, the sheet is transported by the transport rollers disposed before a registration rollers to bring the leading edge of the sheet into abutment with the registration roller at a stop and to press the sheet against the registration roller so that the sheet is curved for registration, the transport rollers disposed before the registration rollers are stopped, and the sheet is held on standby until the registration rollers are activated in synchronism with the rotation of the photosensitive drum. Since the sheet must be held on standby, it is impossible to feed the sheets at short intervals.
The sheet feeder disclosed in the second reference absorbs variations in the time when the sheet arrives at the registration rollers due to the dislocation of the sheet from the sheet stacking position or variation in the transport speed by temporarily stopping the sheet before the transport rollers. Therefore, the time gap at which the sheets are delivered to the photosensitive drum is dependent on a time gap for which the transportation of the sheet is interrupted and hence the time gap at which the sheets are delivered to the photosensitive drum cannot be reduced below the time gap for which the transportation of the sheet is interrupted. In FIG. 4, distance from a reference point is measured upward on the vertical axis and time is measured to the right on the horizontal axis, thick solid lines indicate the respective positions of the leading edge and the trailing edge of a sheet B, thin solid lines indicate the trailing edge of a sheet A preceding the sheet B and the leading edge of a sheet C succeeding the sheet B, broken lines indicate a sheet fed out at the earliest time, alternate long and short dash lines indicate a sheet fed out at the latest time, a2 is the range of variation in the time when the sheet is fed out, b2 indicates a time gap between the sheets at a position immediately before the registration rollers, c2 is the standby time period for which the sheet is held at the registration rollers for registration and for synchronizing the operation of the registration rollers with that of the photosensitive drum, and d2 (=b2+c2) is a time gap between the trailing edge of one sheet and the leading edge of the next sheet being delivered to the photosensitive drum. Since the sheet is stopped temporarily at a position immediately before the transport rollers to absorb variations in the time when the sheet is fed out, the sheet arrives at the registration rollers at a fixed time regardless of variations in the time when the sheet is fed out. The gap between the successive sheets decreases to a minimum at the position immediately before the transport rollers. If the gap between successive sheets is zero when the successive sheet is fed out the earliest possible time, the time gap b2 is equal to the difference between the range a2 and the standby time c2, and hence the time gap d2 at which the successive sheets are delivered to the photosensitive drum is equal to the range a2; that is, the minimum time gap at which the sheets are delivered to the photosensitive drum is dependent on the maximum transportation interruption time gap, and the minimum time gap cannot be reduced below this maximum transportation interruption time gap.